After a successful launch of a new communications satellite, it is essential to test the communication's subsystem while a spacecraft is in orbit so as to compare with prelaunch data in order to ensure that no impairment has resulted from the stress of the launch and to verify that the spacecraft payload is compliant with the specification sought.
As to the in-orbit test technology it has to be considered that the spacecraft has to be operational very quickly while not sacrifying the number of tests that have to be performed. Thus, microwave measurement techniques with more powerful computers and software technology have been used to automate the beginning of life test of a new communications satellite.
In order to give an understanding of the tests which have to be performed, it has to be noted that a communication channel is in general separated in adjacent channels with the help of adequate filters. The channel filters fulfill two distinctive tasks: in first place, the specific channel filters have to avoid an interference from and to adjacent channels, and secondly, the signal passing the channel must not be subject to distortion due to filter characteristics.
In the following, a satellite transponder as a communication channel is described more in detail with regard to FIG. 1.
A transponder of a communication satellite comprises a receiving antenna 1 for receiving an uplink signal sent from a ground station (not shown). An output signal of said receiving antenna 1 is fed to an input demultiplexer (IMUX) 3 after frequency conversion in frequency converter 2. Said input demultiplexer 3 comprises several first filters 4-1 to 4-n for separating individual signals within the signal from the antenna. Typically, one filter is provided for each signal to be separated from the other signals received via said receiving antenna 1 and corresponds to a communication channel. The n output signals of said input demultiplexer 3 are fed to a corresponding number of high power amplifiers 5-1 to 5-n in each of which a traveling wave tube (TWT) is employed for amplifying the output signals of said input demultiplexer 3. As each of said high power amplifiers is normally operated in its saturation point, multiple signals would create intermodulation products and distortion of the signals. The amplifier output signals are passed through second filters 6-1 to 6-n which are part of an output multiplexer (OMUX) 7 combining the n amplifier output signals. The output signal of said output multiplexer 7 is fed to a transmitting antenna 8 for being transmitted to the desired area on the ground.
Hence, the demultiplexer (IMUX) consists of one filter per channel which separates the wanted signal from all other incoming signals. This separation is necessary in order to avoid multiple signals to reach the respective high power amplifier. Each channel has its own assigned high power amplifier (generally a traveling wave tube amplifier). As these high power amplifiers are normally operated in their saturation point, multiple signals would create intermodulation products, and thus distort the signals at the output of the satellites transmit antenna (down-link).
After the amplification, the signals are passed through a second filter (at the output multiplexer OMUX filter) which should avoid the broadband noise of the amplifiers to interfere with adjacent channels. Before being transmitted via the satellite's transmit antenna, all signals are combined again in the output multiplexer (OMUX).
The transponder filter characteristics are thus mainly determined by the IMUX and OMUX filters, which are in general realized in waveguide technology. Prior to launching a satellite, the filters are designed and tested according to specifications. After the launch, the entire satellite, including the payload and especially the filters have to be tested in order to verify if during the launch phase, filters have been damaged. The beginning of life tests are is thus a very crucial part before the operational life of a satellite.
For the beginning of life tests, the satellite is in general placed at an orbital position, which is not the final destination of the satellite. The reason for this is for instance that if the satellite contains transponders (backup) which may interfere with operational transponders, the measurement signal on the satellite under test may create interference to payload signals of the operational satellites.
The orbital position on which a new satellite is placed during beginning of life tests is in general coordinated in such a way that no interference is created to any operational system. However, as the geostationary arc is more and more crowded with Ku-band satellites, it becomes more and more difficult to identify a slot, in which all bands of the satellite under test may be measured with high power signals.
Hence, when planning in-orbit tests on a communication satellite, a major constraint is to ensure that no radio interference can be generated, disturbing other communications satellite systems sharing the same frequency bands and, conversely, that the results of the satellite under in-orbit testing are not jeopardized by transmissions associated with other systems.
From the publication in International Journal of Satellite Communications, Vol. 13, 403-412 (1995); C. Moens and F. Absolonne: “ESA's in-orbit test facilities for communications satellites” those aspects were analyzed by a in-orbit test plan to cope with the following situations of the “maritime satellite based on the European communication satellite system bus”—program:
(1) The presence of other maritime satellites resulted in non-permissible frequency slots in which it was not allowed to generate a carrier from the ground or from the satellite is payload, e.g. the TDMA access channel and the search and rescue frequency band. This was accommodated in the software of the in-orbit testing computer by creating a frequency plan which inhibited use of the non-permissible frequency slots, such that test signals can only be generated in the permitted frequency channels.
(2) The second problem was created by the forward transponder containing an automatic level control function which keeps the output power constant irrespective of the transponder loading. The consequence is that the transmitted noise power increases, as transponder loading decreases, to reach an unacceptable level (from the interference point of view) when the transponder is not loaded. This difficulty has been overcome by ensuring a permanent minimum loading of the forward transponder. The loading was created by automatically up-linking two carriers, generated by dedicated synthesizers, during the inactive periods of the payload commissioning and acceptance phases. In the case of a malfunction in the C-band up-link during any in-orbit test, the minimum loading violation was detected by a specific computer program activating the loading carriers and aborting the ongoing test. The last result was for the European Space Operations Center to switch off the satellite payload.
Hence, according to conventional techniques the characteristics as amplitude response and group delay of the IMUX and OMUX filters is performed with a microwave link analyzer, which uses a frequency modulated carrier to measure amplitude response and group delay. The microwave link analyzer determines the group delay at a specific frequency by differentiating of the phase delay over frequency. As in a channel using a high power amplifier like a TWTA, the amplitude to phase modulation (AM/PM) conversion can lead to erroneous measurements, the power of the MLA signal has to be far below the saturation point.
U.S. Pat. No. 5,546,421 discloses a self-compensating spread spectrum hybrid which is used in a communication station coupled to a bi-directional input-output signal path. Such a communication station could be e.g. a simple terminal equipment of a telephone line. A hybrid circuit is defined as a multi-port component that roots an incoming signal to a neighbouring port without influencing the other ports. This property is called “directivity”, wherein the quality of the directivity is characterized by the “isolation”. The maximum isolation is achieved when the bi-directional signal transmission path presents an impedance to be bi-directional signal port of the hybrid which matches the impedance for which the hybrid is designed. U.S. Pat. No. 5,546,421 identifies the problem that the impedance presented by the bi-directional signal path to the hybrid may change dynamically during operation. As a solution it is suggested that the impedance of the bi-directional line is measured by a S11-measurement using a spread spectrum technique. The spread spectrum pilot signal covers the bandwidth of the information signal, and is reflected in an amount and with a phase which depends upon the relative impedance presented to the hybrid by the bi-directional signal path. The receive signal, which arrives at the station bi-directional port from the bi-directional path, is coupled from the hybrid to a spread spectrum demodulator, which regenerates the own station pilot signal with a phase which depends upon the impedance presented by the bi-directional signal path to the hybrid. The pilot signal is phase-detected, processed and applied to the hybrid to minimize the amount of transmit signal coupled to the receive board in a closed-loop operation.
U.S. Pat. No. 4,637,017 discloses a method of measuring input-back off to an amplifier in an time-division multiple access (TDMA) communication system having a carrier recovery segment and a clock recovery segment prior to a data segment in traffic burst. A monitoring station transmits a CW pilot signal within the amplifier's bandwidth. In the guard time between bursts, the monitoring station measures the unsuppressed pilot level output by the amplifier. While a ground station is transmitting an unmodulated carrier during carrier recovery or a carrier modulated at the clock frequency during clock recovery, the monitoring station measures the suppressed pilot signal. The amount of pilot suppression is related to the input power back off of the carrier by a previously measured or a theoretically derived relationship. The carrier-to-suppressed noise ratio is determined by measuring the carrier level during the carrier recovery and by measuring the suppressed noise during carrier or clock recovery during a noise filter centered away from any transmitted signals or their intermodulation products.
After a successful launch of a new communications satellite, various satellite subsystems have to be tested and their performance has to be evaluated. The most important drawback of the conventional testing techniques is that the measurement signal can create interference to an adjacent satellite system. On the other hand, the measurement signal itself is subject to noise and interference created by signals coming from adjacent satellites. This situation is explained in more detail below according to FIG. 5.